Biological creatures with unique surface wettability have long served as a source of inspiration for scientists and engineers. More specifically, certain beetle species in the Namib Desert have evolved to collect water from fog on their backs by way of wettability patterns, which attracted an ongoing interest in biomimetic studies. Bioinspired materials exhibiting extreme wetting properties, such as superhydrophilic and superhydrophobic surfaces, have attracted considerable attention because of their potential use in various applications. Combining these two extreme states of superhydrophilicity and superhydrophobicity on the same surface in precise two-dimensional micropatterns opens exciting new functionalities and possibilities for a wide variety of applications. In this review we briefly describe the water-harvesting mechanisms of a genus of Namib Desert beetle, Stenocarpa, consisting of the theory of wetting and transporting. Then we describe the methods for fabricating superhydrophilic-superhydrophobic patterns and highlight some of the newer and emerging applications of these patterned substrates that are currently being explored. Finally, we provide conclusions and outlook concerning the future development of bioinspired surfaces of patterned wettability.

All living creatures in the world, including human beings, require water for their survival. However, over the past few decades, water shortage has been one of the most serious global crises, which greatly threatens the survival of billions of people in some arid and developing countries (Gao & You 2015; Hybel et al. 2015; Chaturvedi et al. 2016; Mekonnen & Hoekstra 2016; Zhu et al. 2016). Actually, water is the most abundant resource in the natural environment, but salt water is almost 96.54% of all the water on the Earth. The freshwater available directly to people is only 0.36%, mainly arising from frozen glaciers, polar ice caps and unfrozen groundwater, of which a small fraction is present on the surface or in the air (Kalogirou 1997; Gohari et al. 2012; Gorjian et al. 2014; Gorjian & Ghobadian 2015). The water problems arising from fast-growing economic activity and poor management of water resources have attracted widespread attention worldwide as they affect human health and slow down economic development (Jurado et al. 2012; Vengosh et al. 2014; Gorjian & Ghobadian 2015; Hanna-Attisha et al. 2016; Ge et al. 2017). Therefore, obtaining clean water is essential for human society as well as maintaining the diversity of our living environment.

How to obtain more water is an issue that needs to be urgently addressed (Ridoutt & Pfister 2010; Ahmad & Deog-hwan 2015; Carroll et al. 2015; Gilbertson et al. 2015). Nowadays, seawater desalination technology has been extensively applied to solve human water shortage problems, to reduce dependence on precipitation cycles and increase the availability of traditionally unusable water. However, although desalination is a reliable technology for creating freshwater, dramatically high costs and reliance on energy are needed for this method, which have limited further application (Fritzmann et al. 2007; Khawaji et al. 2007; Lee et al. 2011; Menachem & Phillip 2011; Zhao et al. 2012; Grubert et al. 2014; Lutchmiah et al. 2014).

Recently, much attention has been paid to metal-organic frameworks (MOFs) – materials which adsorb water from the air (Alina et al. 2019; Suh et al. 2019; Hanikel et al. 2020; Logan et al. 2020; Pan et al. 2020). MOFs are a new type of porous material, connected by metal nodes and organic ligands, which have unprecedentedly high surface areas and porosities (Li et al. 2019). Although they have great potential to collect water from the air, the application of these materials in the field of water harvesting presents some challenges as most MOFs have poor abilities to withstand structural degradation in the presence of water or vapor (Schoenecker et al. 2012). However, with the deepening of the research, MOFs materials will definitely play an important role in the field of water collection in the near future.

How to collect water from the air in dry areas

Increasing biomimetic research shows that the solution to the global problem of water availability could be found in nature (Schemenauer & Cereceda 1991; Schemenauer & Cereceda 1994; White et al. 2013; Chen et al. 2018; Zhou et al. 2018). Indeed, over 4.5 billion years of evolution and natural selection, natural creatures have evolved to take advantage of minimum resources to achieve maximum function (Woodyer et al. 2004; Peisajovich 2012; Gou et al. 2014). In regions where water has been a limited resource for centuries, nature has adapted to provide survival methods for species inhabiting the ecosystems concerned. For example, certain beetle species in the Namib Desert have evolved to collect dew and fog from the air on their backs by way of wettability patterns (Parker & Lawrence 2001). This has been described from the study of the elytra of beetles from the genus Stenocara and has attracted increasing interest in biomimetic studies (Ju & Zheng 2014; Chen et al. 2018; Comanns 2018; Li & Guo 2018). Fog brings water in the form of minute droplets and fog events occur on approximately 30 days per year in the Namib Desert, which therefore represent a predictable source of water for the Namib Desert beetles (Seely 1979; Lancaster et al. 1984; Pietruszka & Seely 1985). In a foggy dawn, the Stenocara beetle tilts its body forward into the wind to capture small water droplets in the atmosphere. After these small droplets coalesce into bigger water droplets, they roll down into the beetle's mouth, providing it with a fresh morning drink. The structures behind this process are believed to be an array of surface bumps topped with hydrophilic spots (about 100 μm in diameter) on a superhydrophobic background; small water droplets carried by the morning fog settle on the hydrophilic peaks of the smooth bumps on the wings of the beetle and form fast-growing droplets; once the weight of a growing droplet is sufficient to overcome the binding forces of the hydrophilic region, it rolls down toward the beetle's head.

With the inspiration of the fog-collection principle of the patterned hydrophilic-hydrophobic surface of desert beetles (Hamilton & Seely 1976; Garrod et al. 2007; Cao et al. 2015; Park et al. 2016), biomimetic materials with smart structures are widely fabricated for water collection (Zhai et al. 2006; Lee et al. 2007; Rizzello et al. 2009; Kang et al. 2010; Piret et al. 2010; Zhu et al. 2010; Li et al. 2012; Zhang et al. 2015). In this review, the recent research on the fog-collecting materials inspired by Namib Desert beetles, particularly on the mechanisms of water capture, the fabrications, functions, applications, and new developments in recent years are summarized. The theoretical basis is first introduced to help understand the wetting behaviors in water collection, and the Young's equation, Wenzel model, Cassie model are described. This is followed by a discussion on the water-collecting mechanism of Namib Desert beetles and their bioinspired materials. Finally, the remaining challenges and promising breakthroughs concerning the future development of bioinspired materials are presented.

Basic theories

Generally, water collection comprises four steps, ‘condensation’, ‘coalescence’, ‘transportation’, ‘absorption’, which are deeply affected by the wetting characteristics and water droplet transportation. To explain the water collection mechanism, researchers have proposed a number of models, including Young's contact, Wenzel state, Cassie state, illustration of contact angle hysteresis. When a droplet of liquid is placed on a smooth solid surface, the apparent equilibrium contact angle can be described by Young's equation as follows (Young 1805):
(1)
where represents the contact angle of Young's model, and γ is the surface tension. As shown in Figure 1(a), , , are the surface tension of the solid in air, the interface energy between the solid and liquid, and the surface tension of liquid in air, respectively. However, this surface is in an ideal smooth state, ignoring the influences of roughness, chemical heterogeneity, surface reconstruction, swelling, dissolution and so on.
Roughness is considered to be an important parameter in terms of water contact. Two models, the Wenzel and Cassie models, are commonly used to discuss the surface roughness with the apparent contact angles. As is shown in Figure 1(b), when a droplet is in contact with a rough surface, it completely fills the valleys of that surface. The water contact angles (CA) on the rough surface are described by the Wenzel models as follows:
(2)

Here, r (r > 1) means the ratio of the real contact line to the projected contact line of the portion of solid in air. It is shown in the equation that the combined effect of surface morphology (r) and the surface chemical composition () are observed to be decisive factors to influence the apparent contact angle. Nevertheless, the absolute value of might be larger than 1 in some surfaces with high roughness or porous structures. So as the Wenzel model is insufficient, the Cassie model will be used to introduce such wetting behavior instead.

Chemical heterogeneities in the surface are considered to be another set of factors that have an effect on the equilibrium contact angle. A composite state is formed, resulting from liquids only being in with contact the solid through the top of the asperities, while air pockets are trapped underneath the liquid. It also demonstrates that the air constructs a perfect non-wetting state as shown in Figure 1(c). The apparent contact angle in the Cassie model can be described as:
(3)
where f is the solid-water fraction under the contact area; shows a lower value than r in the Wenzel model and is defined as the roughness ratio of the wet part of the solid surface. The solid-water fraction f (solid-air fraction), similar to r, is also suggested as a parameter that influences the apparent contact angle.
For the dynamic wettability evaluation of a liquid-repellent surface, Furmidge proposed the Furmidge equation to explain the dynamic wettability as follows (Furmidge 1962):
(4)
where m, and w are the weight, surface tension and contact circle width of the liquid droplet, respectively, g is the gravitational acceleration, the sliding angle, and and the advancing contact angle and receding contact angle, respectively (Figure 1(d)). In the Furmidge equation, the significant characterization parameters are contact angle, sliding angle or contact angle hysteresis (the difference between the advancing contact angle and receding contact angle ). Contact angle indicates the degree of water-repellency of liquid on the solid surface; whereas a low sliding angle or small contact angle hysteresis reveals that the droplet has low adhesion and can slide readily on the surface.
However, gravity can be ignored if the droplet size is smaller than the capillary length (, where g is the gravitational acceleration and is the density of the liquid). Thus, on most occasions of spontaneous directional transportation, the main resistance is the contact angle hysteresis, which is mainly caused by inhomogeneity on the surface and can be calculated as follows (Bai et al. 2010; Lv et al. 2014):
(5)
where is the contact radius and and are the receding and advancing contact angles, respectively (Figure 1(e)).

Water-collecting mechanisms

The collection of water by bioinspired micropatterns is becoming increasingly important and has attracted increasing attention. To explain the water collection mechanism, researchers have proposed many models in recent years, such as driving forces, resistance forces and hanging ability. These models can help people to have a deep understanding of the process of the collection of water by bioinspired microfibers and direct the design of bioinspired microfibers for better water collection.

Fog collecting

As one of the world's oldest and driest deserts, the Namib Desert formed 80 million years ago and is now located in the Namib-Naukluft Park in Africa (Geyh & Heine 2014; Hamdan et al. 2015). The annual rainfall is less than 13 mm (0.5 inch), although at night, fog coming from the sea brings some water, which is vital for the native flora and fauna to survive in (Jankowitz et al. 2008; Murray et al. 2016). After thousands of years' evolution, a host of animals are well adapted to the arid conditions.

The Namib Desert beetle is distinguished by its outstanding water-collection ability (Norgaard & Dacke 2010). In 2001, Parker and Lawrence (Parker & Lawrence 2001) reported the water capture on the surface of a desert beetle. The Stenocara beetle's back is covered with an array of non-waxy hydrophilic bumps surround by a wax-coated hydrophobic background (Figure 2(a)), which is supposed to facilitate water formation and absorption. On a foggy morning, the fog is mainly blown in from the Atlantic Ocean; the fog droplets settle and get trapped on the hydrophilic seeding patches with the contact angle θ1, which prevent them from being lost to the winds and heat of desert, because the fog droplets with diameter 1−40 μm are much finer than raindrops. Water can also be collected when it comes into contact with the superhydrophobic region with the contact angle θ2, as droplets may rebound or blow into the hydrophilic area (Figure 2(b)). The size of the attached droplet within the hydrophilic path grows gradually due to coalescence. When an attached droplet eventually reaches a size at which its contact area covers the entire hydrophilic island and the front contact angle reaches θ2, the front contact line of the droplet starts to move across the boundary between the two domains with different wettabilities (Oliver et al. 1977; Chang et al. 2009; Chang & Hong 2010). Dorrer & Ruhe (2008) presented a quantitative analysis of the critical volumes () at which drops dewet from the bumps when the substrates are tilted at a fixed angle (). Generally, for an inclined drop staying at rest on a surface of homogeneous wettability, there exists a capillary pinning force against the gravity of water droplets. The pinning force is proportional to liquid-gas interfacial tension, characteristic length, and the difference between the receding and advancing contact angles. According to the classical MacDougal-Ockrent-Frenkel (MOF) approach, the critical volume () of a stable drop can be estimated by balancing the maximum pinning force with the gravitational force as follows:
(6)
where the characteristic length d is normally defined as drop width, the liquid-gas interfacial tension, the liquid density, and g the gravitational acceleration. and are the receding and advancing contact angles at the uphill and downhill edges of the contact line, respectively.
Figure 1

Different wetting behaviors of a water drop on a solid surface: (a) Young's model, (b) Wenzel's model, (c) Cassie's model, (d) the Furmidge model, (e) illustration of contact angle hysteresis.

Figure 1

Different wetting behaviors of a water drop on a solid surface: (a) Young's model, (b) Wenzel's model, (c) Cassie's model, (d) the Furmidge model, (e) illustration of contact angle hysteresis.

Close modal
Figure 2

(a) The water capturing surface of a Namib Desert beetle's back at different scales (Parker & Lawrence 2001). (b) Schematic diagram of small water droplets changing over time.

Figure 2

(a) The water capturing surface of a Namib Desert beetle's back at different scales (Parker & Lawrence 2001). (b) Schematic diagram of small water droplets changing over time.

Close modal
Once the critical volume () has been reached, the droplet detaches and rolls downward into the beetle's mouth. By this stage, the droplet is sufficiently massive to roll into the wind, its cross-section (the area of contact with the wind) having increased far more slowly than its volume. Using the control volume form of Euler's first law, it is possible to estimate the windspeed, v, below which the droplets will roll downwards:
(7)
where is the density of a medium (), R is the droplet radius, g is the gravitational constant (9.8 ) and is the tilt angle. The equation predicts that the diameter of a spherical droplet must exceed 5 mm if it is to roll down a 45° incline into a headwind of 5 (as measured in Namibia). This is fairly consistent with video footage of the beetles in their natural environment, in which droplets of roughly 4–5 mm diameter form in a steady, self-replenishing stream.

Dew condensation

It is worth noting that fog collection and vapor condensation might not be the same process. Fog harvesting depends on the collision between the tiny fog droplet and the fog-collecting surface, which is irrelevant to phase transition (Cao & Jiang 2016). Dew is the condensation of water vapor into liquid droplets on a substrate; it is a common natural phenomenon and considered to be the inverse of evaporation (Beysens 2006).

In order to determine whether dew water collection is also possible for desert beetles, Guadarrama-Cetina and Mongruel (Guadarrama-Cetina et al. 2014) investigated the infra-red emissivity, and the wetting and structural properties, of the surface of the elytra of a preserved specimen of the beetle. They performed scanning electron microscopy on histological sections and determined the infra-red emissivity using a scanning pyrometer; the emissivity measured was close to the black body value. The results showed that the characteristics of these beetle elytra prompt dewdrop formation.

While fog basking collects airborne droplets which are already in a liquid state and carried in the wind, harvesting water from unsaturated air, which has more general implications, requires a different approach. That is, humid air needs to be cooled down to the dew point for water vapor to condense into liquid water. The growth of water droplets on a plane surface follows different rules (Briscoe & Galvin 1906; Medici et al. 2014). In the time sequence of the formation of a liquid droplet, the first event is the formation by thermal fluctuations of the smallest nucleus. However, the energy of formation of the liquid-vapor interface is an energy barrier to cross (Vekilov 2010). According to Volmer's classical nucleation theory, the free energy barrier for the formation of a liquid nucleus on a flat surface depends strongly on the intrinsic wettability of the surface via the contact angle :
(8)
where is the liquid-vapor surface energy and r is the critical radius. The formula for the critical radius is given by Kelvin's classical equation as:
(9)
where p is the vapor pressure over a curved interface of radius , is the equilibrium vapor pressure above a flat surface of the condensed phase at temperature T, is the number of molecules per unit volume of the liquid, and k is the Boltzmann constant. The intrinsic wettability of the surface has a strong effect on the nucleation rate J via the inverse exponential dependence on :
(10)
where is a kinetic constant. The nucleation energy barrier continuously increases with contact angle, indicating that hydrophobic surfaces have higher when compared to hydrophilic surfaces under identical conditions. Consequently, the nucleation rate on the hydrophilic surface would be significantly higher than that on the hydrophobic surface. Hence, nucleation would be favored on the hydrophilic regions of the surface patterned with hydrophobic and hydrophilic regions. The greater the intrinsic wettability difference between these regions, the stronger is the propensity to cause this preference. The beetle's surface can capture water by direct and preferential heterogeneous vapor-to-liquid nucleation onto the hydrophilic regions of the surface. Subsequently, these droplets grow by further condensation and coalescence and roll into the beetle's mouth, and these growth laws were indeed observed on both valleys and bumps (Varanasi et al. 2009; Guadarrama-Cetina et al. 2014). Thus, we believe that beetle's surface is nature's version of a dropwise condensing surface.

Beetles in the Namib Desert have unique morphologies and surface microstructures for highly efficient water collection. Motivated by the Stenocara beetle, many hydrophilic-hydrophobic patterned surfaces have been constructed. Generally, on such patterned wettable surfaces, there are at least two portions with different wettabilities ((super)hydrophilicity and (super)hydrophobicity). Next, we briefly introduce the main existing techniques for the fabrication of bioinspired hydrophilic-hydrophobic patterned surfaces.

Photolithography technology

Zahner et al. took advantage of the selective illumination of UV light via a photomask to fabricate superhydrophobic-superhydrophilic patterned porous polymer films (Zahner et al. 2011). The method they adopted was based on the preparation of a hydrophobic thin porous polymer film, which was then modified by UV-initiated surface photografting through a photomask (Ranby 1992; Rohr et al. 2003). In order to create a superhydrophilic micropattern with defined geometry on the superhydrophobic polymer background, they first prepared superhydrophobic, microporous poly(butyl methacrylate-co-ethylene dimethacrylate) films by UV-initiated radical polymerization of a prepolymer mixture containing monomers, porogens, crosslinkers, and a UV initiator. Then the film was wetted with a photografting mixture composed of a methacrylate monomer, benzophenone as the initiator, and a mixture of tert-butanol and water. Finally, the polymer surface was irradiated with UV light through a photomask.

Such patterns with extreme differences in the wettability between superhydrophilic and superhydrophobic areas can be applied to microfluidic channels, water collection, cell growth and site-selective immobilization of functional materials (Wu & Zhang 2015; Tserepi et al. 2016; Yu et al. 2017; Kostal et al. 2018). To further develop such functional interfaces, it is critical to enhance contrast in the wettability on the patterned surfaces. Nishimoto et al. prepared a superhydrophobic surface with an extremely high static water contact angle by surface modification with self-assembled monolayers (SAMs) of octadecylphosphonic acid on rough nanostructured anatase TiO2 surfaces (Nishimoto et al. 2014). The superhydrophobic TiO2 films could be conveniently patterned with UV light to produce superhydrophobic-superhydrophilic patterns with an extremely high wettability contrast (∼170°), which could be employed to selectively fill superhydrophilic areas with water-based functional materials.

Additionally, Bai et al. designed a novel kind of surface with star-shaped wettability patterns to improve the water-collection efficiency with control over the surface wettability. By integrating Laplace pressure gradient and surface energy gradient, the patterned surface can quickly drive tiny water droplets toward more wettable regions to avoid them being lost in the wind. The results showed that this type of surface is more efficient in water collection than uniform superhydrophilic, uniform superhydrophobic, or even circle-patterned surfaces.

Surfaces with star-shaped wettability patterns can be fabricated following the procedures in Figure 3. First, they fabricated a superhydrophilic surface by depositing TiO2 slurry onto a glass substrate via a spin-coating method, then the film was treated with fluorinated alkyl silane (FAS) to change the wettability from superhydrophilic to superhydrophobic. Subsequently, the circular-shapes pattern and 4-, 5-, 6-, 8-pointed star-shapes of photomask were utilized to construct the superhydrophilic pattern on superhydrophobic surface via selected exposure to UV light. Water-collection experiment results showed that the shape and size of the pattern is crucial for enhancing the water-collection efficiency on patterned surfaces. These investigations may provide insights in designing and developing materials with controllable wettability for highly efficient water- or liquid-collecting technology (Bai et al. 2014).

Figure 3

(a)–(c) Schematic illustration of the fabrication process of bioinspired surfaces with star-shaped wettability and the corresponding images of condensate droplets wetting state. (d) Top view and side view scanning electron microscope image of the spin-coated TiO2 film and optical image showing (e) the relationship between the wetting property of an FAS-modified TiO2 film surface and UV illumination time. (f) Different fog-collecting processes on surfaces with various wettability features. (g) Schematic illustration of the method used to quantitatively measure the fog-collection efficiency of different surfaces. (h) Fog-collecting efficiency of different wettability surfaces. (i) Comparison of efficiency between five-pointed star-shaped patterns and circle-shaped patterns when the substrate is inclined at 15°, 45°, 90°. (j) Ratios of star-shaped patterns to circle-shaped pattern. Reproduced with permission (Bai et al. 2014).

Figure 3

(a)–(c) Schematic illustration of the fabrication process of bioinspired surfaces with star-shaped wettability and the corresponding images of condensate droplets wetting state. (d) Top view and side view scanning electron microscope image of the spin-coated TiO2 film and optical image showing (e) the relationship between the wetting property of an FAS-modified TiO2 film surface and UV illumination time. (f) Different fog-collecting processes on surfaces with various wettability features. (g) Schematic illustration of the method used to quantitatively measure the fog-collection efficiency of different surfaces. (h) Fog-collecting efficiency of different wettability surfaces. (i) Comparison of efficiency between five-pointed star-shaped patterns and circle-shaped patterns when the substrate is inclined at 15°, 45°, 90°. (j) Ratios of star-shaped patterns to circle-shaped pattern. Reproduced with permission (Bai et al. 2014).

Close modal

Moazzam et al. utilized the negative photolithography and biopolymer of polydopamine (PDA) method to produce a porous membrane surface with contrasting wettabilities by creating hydrophilic patterns (nanoscale PDA-coated SU-8 bumps) on the hydrophobic background of polypropylene (PP) membranes (Moazzam et al. 2018). The fabrication process involves four main steps as shown in Figure 4. First, the PP microfiltration membrane (PPMM) was bound to a silicon wafer using positive photoresist as an intermediate adhesion layer. This step was followed by spin coating an approximately 200 um thick SU-8 layer on the PPMM surface. In order to make the SU-8 surface superhydrophilic instead of hydrophobic surface with the contact angle of 90°, they used a PDA coating which reduces the free energy of the surface and creates an SU-8 surface with contact angle of 10°. The next step was to transfer the mask geometry to the SU-8 photoresist surface using a negative lithography process. After that, the sample was immersed in acetone to separate the PPMM from the silicon wafer and obtain the product. They then investigated the fog-harvesting performance of different surfaces and found that the patterned coated SU-8 surface exhibited excellent performance.

Figure 4

(a) Schematic diagram of manufacturing scheme. (b) Morphology of the structure of samples at different magnifications. (c) The roughness and surface topography of the silicon wafer and created bumps on the background surface. They were studied by atomic force microscopy in two and three dimensions. (d) The average roughness (nm) of these samples. (e) Water contact angle (WCA) and surface free energy (SFE) for PPMM membrane, uncoated SU-8 and PDA-coated SU-8. (f) Water-collection rates on PPMM membrane surface, uncoated SU-8 surface, PDA coated SU-8 surface and patterned coated SU-8 surface (Moazzam et al. 2018).

Figure 4

(a) Schematic diagram of manufacturing scheme. (b) Morphology of the structure of samples at different magnifications. (c) The roughness and surface topography of the silicon wafer and created bumps on the background surface. They were studied by atomic force microscopy in two and three dimensions. (d) The average roughness (nm) of these samples. (e) Water contact angle (WCA) and surface free energy (SFE) for PPMM membrane, uncoated SU-8 and PDA-coated SU-8. (f) Water-collection rates on PPMM membrane surface, uncoated SU-8 surface, PDA coated SU-8 surface and patterned coated SU-8 surface (Moazzam et al. 2018).

Close modal

In actual applications, the patterned area using the conventional fabrication method is not suitable for mass fabrication. So, it is desirable to find a method for continuous fabrication of the surface that mimicks the Stenocara beetle's back. Lee et al. proposed a continuous fabrication method to make a bioinspired patterned surface by using roll-type photolithography for potential applications to real-time air-monitoring systems (Lee et al. 2014). The fabrication procedure consists of four detailed processes as follows: molding, deep UV etching, surface treatment, and roll-type photolithography for selective exposure. Low contact-angle hysteresis is not suitable for a water-collecting surface because the water droplets roll away before they get bigger by aggregating and the wettability of solid surface can be controlled by surface topography and/or surface chemistry (Choi et al. 2008). So they tested various materials and micro/nanogeometries to obtain optimized water-contact characteristics for a patterned surface. Water-collection experiments showed that the hydrophilic surface array with a width of 3 mm and a spacing of 5 mm on the hydrophobic surface demonstrated a maximum water-collection performance when compared with other cases. A significant feature of this method is that it allows rapid, permanent changes in surface wettability from superhydrophobic to superhydrophilic to define local wettability without complex time-consuming processing.

However, due to the photocatalytic degradation of low-energy hydrocarbon groups or the corresponding micro/nanostructure damage and other possible circumstances, the practical applications of patterned surfaces with hybrid wettability are likely to be affected for high-efficiency water harvesting. What is more, the bioinspired materials usually function well in the laboratory, completely ignoring the multiple factors in the external environment, let alone the continuous mass production; therefore, how to produce hydrophilic and hydrophobic patterned surfaces in a simple and efficient production method is what we should consider in the future.

Composite technology of materials with different wettabilities

Generally, on such a patterned wettable surface, there are at least two portions with different wettabilities. One possibility for making a composite surface with pattern dimensions is simply pressing together materials with different wettabilities. Based on this method, Cao et al. successfully constructed a Janus system by compositing hydrophilic cotton absorbent and hydrophobic copper mesh, demonstrating an enhanced efficiency for water harvesting (Cao et al. 2015). They first selected the hydrophobic mesh with a smaller pore diameter (<50 um) and the hydrophilic cotton absorbent was selected due to its high potential water capacity, cost efficiency and loose structure. Then the processed Janus system also possessed an intrinsic surface pattern, i.e., the hydrophilic cotton in the pores surrounded by the hydrophobic copper wires. Their experiments with collecting water showed that the hydrophilic cotton absorbent only collected 0.14 ± 0.02 g water in 5 min. The Janus system can gather up to as much as 0.310.03 g water, 1.3 times higher than that of a hydrophobic mesh, showing an excellent water-collecting ability. By further improvement, they designed and fabricated a cylindrical Janus fog collector, changing the 2D hydrophobic-hydrophilic cooperative system into the 3D ones. The device works efficiently no matter which direction the fog comes from.

Similarly, Yin et al. prepared a hybrid wettable surface by incorporating superhydrophobic copper mesh and pristine hydrophilic copper sheet as shown in Figure 5. Firstly, the polytetrafluoroethylene (PTFE) sheet was adhered under the copper mesh and then the resulting sample was fixed on a translation stage in air. Subsequently, the resulting sample was treated with a femtosecond laser and the ejected PTFE nanoparticles in sizes from tens to hundreds of nanometres adhered to the copper mesh. Finally, pristine hydrophilic copper sheet was tightly adhered. It was found that the surface exhibited fog-harvesting properties with good efficiency and the water-collection rate of the as-prepared surface could be optimized by controlling the mesh number and inclination angle. In addition, the as-prepared samples showed anticorrosion properties during corrosion testing (Yin et al. 2017).

Figure 5

(a) The preparation procedure for the hybrid micro/nanopatterned surfaces. (b) Dynamic fog-collecting behavior on a vertically orientated original mesh-sheet sample and prepared sample. (c) Water-collection rates of the different samples. Water-collection rates of the as-prepared samples with different characteristics: (d) sample with different mesh numbers, (e) sample at different inclined angles (Yin et al. 2017).

Figure 5

(a) The preparation procedure for the hybrid micro/nanopatterned surfaces. (b) Dynamic fog-collecting behavior on a vertically orientated original mesh-sheet sample and prepared sample. (c) Water-collection rates of the different samples. Water-collection rates of the as-prepared samples with different characteristics: (d) sample with different mesh numbers, (e) sample at different inclined angles (Yin et al. 2017).

Close modal

Meanwhile, Gao et al. firstly introduced a hydrophilic-superhydrophobic patterned weft-backed woven fabric (Liu et al. 2016; Gao et al. 2018a, 2018b) fabricated by a facile weaving method with simple textile equipment. Hydrophilic viscose and hydrophobic PP yarns were used to produce the hybrid wettable surface with common commercial agents, which sharply relieved the cost, making it possible to produce the water-collecting materials at a large scale in future (Figure 6). Pre-cleaned viscose yarns and PP yarns were finished by dipping in a hydrophilic agent (60 g/L) and a hydrophobic agent (60 g/L) for 30 min each and baking at 120 °C for 10 min. After being dried in an oven, the hydrophilic viscose yarns were used as the weft yarns and the hydrophobic PP yarns were used as the warp yarns. They wove hydrophilic-superhydrophobic patterned surfaces in proportions of 1:1, 1:3, 1:5, 1:7 (viscose yarns:PP yarns) while the other side was completely superhydrophobic with the semi-automatic loom. When the surface area of all the samples was the same, it turned out that the as-prepared sample featured the best water-harvesting rate of 1,267.5 mg h−1 cm−2 at a proportion of 1:1. More importantly, the fabric could be recycled for 10 times: the weft backed woven fabric remained intact even after 2,000 abrasion tests, and water contact angles exceeded 140° on hydrophobic regions and remained at 0° on hydrophilic regions respectively (Gao et al. 2018a, 2018b).

Figure 6

(a) Schematic diagram of preparation of hydrophilic-superhydrophobic patterned weft-backed woven fabric inspired by Namib Desert beetles. (b) Water contact angle and rolling-off angle on the superhydrophobic area of the hybrid wettable surface; practical water-harvesting situation on the sample. (c) Water-harvesting rate of all samples over a period of 4 hours. (d) Water-harvesting rate of all samples. (e) Water-harvesting rate of the sample with ratios of 0:1, 1:7, 1:1 (viscose yarns:PP yarns) with inclinations from 10° to 90° (Gao et al. 2018a, 2018b).

Figure 6

(a) Schematic diagram of preparation of hydrophilic-superhydrophobic patterned weft-backed woven fabric inspired by Namib Desert beetles. (b) Water contact angle and rolling-off angle on the superhydrophobic area of the hybrid wettable surface; practical water-harvesting situation on the sample. (c) Water-harvesting rate of all samples over a period of 4 hours. (d) Water-harvesting rate of all samples. (e) Water-harvesting rate of the sample with ratios of 0:1, 1:7, 1:1 (viscose yarns:PP yarns) with inclinations from 10° to 90° (Gao et al. 2018a, 2018b).

Close modal

Additionally, dewetting of thin polymer films has been extensively investigated as a way to generate patterned surfaces. In the last decade, the Neto group has used dewetting of polymer films extensively to generate patterned surfaces for a range of applications, including atmospheric water capture and cell patterning (Thickett et al. 2011; Telford et al. 2012; Manuel et al. 2014; Al-khayat et al. 2017; Telford et al. 2017). As sol-gel films are much more robust than polymer films and can withstand high working temperature and UV irradiation (Lee & Crayston 1993; Brusatin et al. 2000; Figueira et al. 2015), Colusso et al. investigated for the first time the dewetting of bilayers of sol-gel films: the top film is hydrophilic (silica) and the bottom one hydrophobic (CH3-silica), with the objective of producing silicate patterned surfaces (Colusso et al. 2019). A hydrophobic sol-gel silica film was first spin coated onto a silicon wafer and then the surface wettability of the sol-gel film was made hydrophobic by adding hydrophobic methyl groups: a CH3-modified hydrophobic silica solution was obtained by hydrolysis and condensation of a mixture of tetraethoxysilane and methyltriethoxisilane with a molar ratio of 1. Subsequently, a freshly made xerogel of silica solution (1 day aged) was spin coated onto the CH3-silica film and then was exposed to ethanol vapor at room temperature. As the xerogel was fresh, the ethanol vapor penetrated the film and reduced the rate of condensation through a series of reactions. Finally, hydrophobic non-wettable substrate dewetted.

Inkjet printing technology

Inject printing technology is an extremely simple, effective and economical way of surface wettability patterning to produce high resolution patterns with complicated shapes (Shimoni et al. 2014; Sun et al. 2015; Lee et al. 2016). Zhang's group adopted the inkjet printing method to construct the superhydrophilic micropatterns on superhydrophobic surfaces. First, they applied a mussel-inspired ink (consisting of an optimized solution of dopamine) directly by inkjet printing to superhydrophobic surfaces. Afterward, microdroplets of the dopamine solution with micropatterns were obtained on the surface and then superhydrophilic micropatterns with well-controlled dimensions were achieved by the formation of PDA via in situ polymerization (Zhang et al. 2015). Similarly, Zhu et al. achieved the hydrophilic-superhydrophobic patterned surfaces by inspiration from mussels and Namib Desert beetles, as shown in Figure 7. They adopted Cassie-state superhydrophobic substrates to synthesize the composite materials, which can quickly transport water away from the surfaces and then improve water-collection efficiencies. The Cassie superhydrophobic copper foils were firstly prewetted by dichloromethane (DCM), immediately followed by dropping the mussels-inspired hydrophilic and bio-adhesive dopamine solution on the treated surface. DCM will ‘cloak’ dopamine due to their different surface tensions. Along with DCM volatility and self-polymerization of dopamine, hydrophilic PDA patterns were constructed on the Cassie superhydrophobic Cu foils, thus obtaining patterned materials. The sample with about 8.0% hydrophilic area exhibited the most efficient fog-harvesting properties, with a water-collection rate of 5.5 mg min−1 cm−2. This method can be widely applied on amounts of superhydrophobic materials in the Cassie state, such as Fe plate, Al plate, cotton fabric, Cu mesh, and Ni foam, etc. The study greatly broadens the preparation of fog-harvesting materials and its practical application in the field of water collection (Zhu et al. 2018).

Figure 7

Schematic illustration of the fabrication of hydrophilic-superhydrophobic materials inspired by Namib Desert beetles and mussels. (a) The Cassie superhydrophobic substrate with high water contact angle and low rolling-off angle. (b) The dichloromethane (surface tension γ = 23.1 mN/m) was dropped on the substrate. (c) The dopamine (γ = 68.5 mN/m) was then dropped onto it. (d) The dopamine was pinned to the surface along with DCM volatility. (e) The self-polymerization of dopamine and solvent evaporation at room temperature. (f) A circular black pattern was constructed on the sample. (g) The processes were cycled four, nine, 16 and 25 times to prepare the surface (Zhu et al. 2018).

Figure 7

Schematic illustration of the fabrication of hydrophilic-superhydrophobic materials inspired by Namib Desert beetles and mussels. (a) The Cassie superhydrophobic substrate with high water contact angle and low rolling-off angle. (b) The dichloromethane (surface tension γ = 23.1 mN/m) was dropped on the substrate. (c) The dopamine (γ = 68.5 mN/m) was then dropped onto it. (d) The dopamine was pinned to the surface along with DCM volatility. (e) The self-polymerization of dopamine and solvent evaporation at room temperature. (f) A circular black pattern was constructed on the sample. (g) The processes were cycled four, nine, 16 and 25 times to prepare the surface (Zhu et al. 2018).

Close modal

Li et al. constructed superhydrophilic micropatterns on a superhydrophobic substrate based on printing an ethanol solution containing a phospholipid onto a superhydrophobic porous polymer surface. The method is compatible with different printing techniques, such as microcontact or inkjet printing. First, the lipid solution was printed in an array pattern onto a thin superhydrophobic porous poly(butylmethacrylate-co-ethylene dimethacrylate) (BMA-EDMA) surface using metal spotting pins. After the lipid array was printed, the polymer substrate was wetted with an aqueous solution, making the created lipid layers on the polymer surface switch from superhydrophobicity to superhydrophilicity. Finally, the surface was dried gently with nitrogen, and the superhydrophilic-superhydrophobic micropatterned surface was obtained. Since lipid layers can also incorporate different bioactive molecules, transmembrane proteins, or other functional lipids, this facile procedure for creating superhydrophilic patterns combined with contemporary printing technology will lead to numerous applications (Li et al. 2012).

Sun et al. also presented a flexible, convenient and low-cost fabrication method for creating superhydrophilic-superhydrophobic patterned surfaces by inkjet printing a sacrificial layer on a superhydrophilic surface. First, a thin aluminum film with hierarchical porous nanostructures was deposited on to a silicon substrate via vacuum deposition. Second, high resolution patterns were inkjet printed on the superhydrophilic surface. The printing ink was a PAA (polyacrylic acid) solution with 30 wt%. After modifying the surface with (FAS) and removing the printed water-soluble deposit, the superhydrophilic-superhydrophobic patterned surfaces were obtained (Sun et al. 2016). Additionally, Jiang's group utilized inkjet printing to fabricate a series of superhydrophobic-superhydrophilic patterned surfaces, which would contribute to the research on droplet transport and water collection in the future (Jiang et al. 2016).

Laser ablation

In production, it is difficult to spin-coat a layer of photoresist on a superhydrophobic surface and hierarchical structures are found to be more efficient for water collection than simple patterns (Ju et al. 2012). So, it is imperative to develop a method that can generate high-resolution and large area hierarchical patterns with controllable surface wettability. Using laser ablation to mimic hierarchical surface morphologies has several advantages. First of all, it is a maskless process, which enables direct writing of arbitrary geometries. Moreover, double-hierarchical surface structures can be easy to fabricate (Jörn et al. 2012). Based on this approach, Kostal et al. presented a novel three-step fabrication method to mimic the Namib Desert beetle's elytra. In the first step, a double-hierarchical surface structure was generated on Pyrex wafers using femtosecond laser micromachining, which made it superhydrophilic (water contact angle <10°). In the second step, a Teflon-like coating was applied to switch the wetting state from superhydrophilic to superhydrophobic (water contact angle >150°). In the last step, selective ablation was used to locally recover the superhydrophilic state. As experiments in an artificial nebulizer setup have shown, such micropatterns enhance the fog-collection efficiency by nearly 60% compared to blank glass. The method they presented enables the functionalization of a broad range of materials, such as glass. This opens up possibilities for fog-collection surface micropatterns, especially in the field of microfluidic and biomedical devices (Kostal et al. 2018).

Wang et al. also constructed hierarchical patterns with modified wettability and desired geometry on a superhydrophobic film via laser direct writing. As shown in Figure 8, a porous superhydrophobic TiO2 surface was fabricated by a hydrothermal method. After that the TiO2 surface was patterned by laser writing at a high resolution of 300 nm. The laser beam removes the surface structures, making the film smoother and relatively wettable. The patterned film can precisely and directionally drive tiny water droplets and dramatically improve the efficiency of water collection with a factor of ∼36 compared with the original superhydrophobic film. Such a patterned film might be an ideal platform for water collection from humid air and for planar microfluidics without tunnels (Wang et al. 2017a, 2017b).

Figure 8

(a) Tiny water droplets were directed to move on tree-shaped hierarchical cones. (b) Optical images of hierarchical cones fabricated on a superhydrophobic surface. (c) Schematic illustration of the fog-collection efficiency of different surfaces. Points 1, 2, and 3 represent the location where the water pipes were placed. (d) Design of the tree pattern. Red arrow represents the driving direction of the water droplets. (e) Patterned film in the process of collecting water. (f) Fog-collection efficiency at locations of 1, 2, and 3 (Wang et al. 2017a, 2017b).

Figure 8

(a) Tiny water droplets were directed to move on tree-shaped hierarchical cones. (b) Optical images of hierarchical cones fabricated on a superhydrophobic surface. (c) Schematic illustration of the fog-collection efficiency of different surfaces. Points 1, 2, and 3 represent the location where the water pipes were placed. (d) Design of the tree pattern. Red arrow represents the driving direction of the water droplets. (e) Patterned film in the process of collecting water. (f) Fog-collection efficiency at locations of 1, 2, and 3 (Wang et al. 2017a, 2017b).

Close modal

Recently, Lu et al. reported a novel facile physicochemical hybrid method that combines femtosecond laser structuring with hydrothermal treatment to create a patterned wettability surface with a well-arranged hierarchical nanoneedle structure. They first created uniform microstructure square arrays on Ti sheets by a femtosecond laser system. Then, Ti sheets with microstructures were hydrothermally treated in an NaOH solution at 220 °C for 24 hours using an electric oven. Next, the surface component was transformed to TiO2 by immersing the hydrothermally treated titanium sheets in 1 mol/L HCl solution for 10 min, washing with distilled water, and annealing at 450 °C for 1 hour in air using a muffle furnace. For the creation of surfaces with special wettability, polydimethylsiloxane (PDMS) liquid was evaporated onto the TiO2 surface in a preheated muffle furnace. Superhydrophobic surfaces or patterned wettability surfaces with hierarchical micro/nanostructures were fabricated by to the modification of PDMS. During the water-collection process under a vapor flow environment, the highest performance was achieved for patterned wettability hierarchical micro/nanostructures, which is 2.2 times that of the untreated Ti surface. Moreover, a uniform water condensation under low humidity (28%) was achieved, which has potential applications in harvesting water from arid environments and in high-precision drop control (Lu et al. 2019).

Hydrophilic-hydrophobic patterned surfaces offer a means of controlling the wetting behavior of aqueous media. This is important for a whole host of technological applications including: cell growth, protein manipulation, the spotting of biomolecules, microfluidics (to control the location and movement of liquids), and the formation of anti-dew/frost-free protective exteriors.

Water collection

Efficient water collection from a humid atmosphere is critical for creatures living in water-limited areas. The Namib Desert Stenocara beetle uses the surface of patterned wettability on its back to collect drinking water from fog-laden wind. The beetle's back consists of alternating hydrophilic bumps and superhydrophobic channels. Inspired by this surface design, Yu et al. fabricated superhydrophilic-superhydrophobic patterned surfaces on the silica poly(dimethylsiloxane) coated superhydrophobic surfaces, to mimic the function of the beetle's back, via a pulsed laser deposition approach with masks (Yu et al. 2017). The wettability patterns exhibit extreme hydrophobic contrast. Water sprayed on superhydrophobic patterns will form spherical droplets. Most of the droplets bounce and roll on the superhydrophobic regions and eventually adhere to the hydrophilic patterns. In order to improve the water-collection efficiency with control over the surface wettability, Bai et al. designed a novel kind of surface with star-shaped wettability patterns (Bai et al. 2014). The patterned surface integrates a surface energy gradient and Laplace pressure gradient. As a result, this type of surface is more efficient in water collection than uniform superhydrophilic, uniform superhydrophobic, or even circle-patterned surfaces. Recently, surfaces with two-tier hierarchical micro/nanostructures are also predicted to promote high-frequency droplet growth and removal due to the existence of their unique structures (Cho et al. 2017; Wang et al. 2017a, 2017b). From the methods described above, we have found that Laplace pressure determined by surface morphology or chemical composition is crucial to water-harvesting efficiency because it can influence tiny water droplet condensation and transportation.

Biomedical

Cell adhesion and proliferation is an important physiological process, which is strongly affected by surface topology and chemistry. Moreover, it is important to control cell adhesion to surfaces for biological studies and diagnosis. It has been reported that superhydrophobic surfaces can completely inhibit cell adhesion, while superhydrophilic surfaces will enhance cell attachment (Song et al. 2009). Cell interactions with superhydrophilic and superhydrophobic surfaces fabricated by patterning have been extensively investigated (Oliveira et al. 2011; Oliveira et al. 2014a, 2014b). Ishizaki et al. successfully fabricated a micropatterned superhydrophobic-superhydrophilic surface by plasma chemical vapor deposition and vacuum ultraviolet irradiation. Physicochemical properties of the surface affect cell adhesion and cell-cell interactions. In particular, mouse 3T3 fibroblast cells attached to the superhydrophilic regions in a highly selective manner while cell adhesion was suppressed on superhydrophobic surface. Moreover, the amounts of the protein adsorption on the flat hydrophilic surface were much greater than those on the flat hydrophobic surface (Ishizaki et al. 2010).

Recently, Popova et al. demonstrated the applicability of the droplet-microarray (DMA) platform based on superhydrophobic-superhydrophilic patterning for cell-based high-throughput screenings (Popova et al. 2016 , 2017). The unique feature of the DMA platform is the ability to form homogeneous droplet arrays spontaneously without the need for the pipetting of each droplet. Droplet arrays are formed spontaneously due to the effect of discontinuous dewetting. Biocompatibility of the polymer surface allowed us to adopt such arrays for cell culturing and create arrays of droplets containing cells. DMA slides enable miniaturized screenings of live cells in droplets ranging from 3 to 80 nL in densities from 588 to 4,563 spots per standard microscope glass slide, which corresponds to approximately 6,000 and 50,000 spots per area of a standard microtiter plate, respectively. The single-step pipetting-free seeding results in savings in the pipetting steps, pipetting tips, robotics, and time of the experiment.

Cell-biomaterial interactions have been widely investigated on flat 2D surfaces; however, studies in a 3D environment are more valuable because they mimic in vivo cell microenvironments better. The dimensionality of the system can influence many cell functions, including polarity, morphology, motility, and cell-cell interactions (Baker & Chen 2012; Dolatshahipirouz et al. 2014). Oliveira et al. successfully used polystyrene superhydrophobic surfaces patterned with wettable spots as improved and versatile platforms for high-throughput spheroids formation and drug screening in such in vitro-constructed tissues (Oliveira et al. 2014a, 2014b). The wettability contrast of the chips was used to fix cell suspension droplets in the wettable regions and evaluated on-chip drug screening in 3D environment. A fibroblast (L929) and an osteosarcoma cell line (SaOs-2) were used for spheroids formation and drug screening studies. It was previously shown that protein adsorption in the wettable regions of the chips is higher than in the superhydrophobic parts. Moreover, cell adhesion and proliferation were also diminished in the superhydrophobic parts of chips made of different polymers (Ueda & Levkin 2013; Dolatshahipirouz et al. 2014).

Liquid transportation

Droplets sitting on surfaces with different wetting states will present different contact areas, contact angles and contact angle hysteresis. Hence, it is possible to control droplet motion and liquid transportation by tuning surface wettability. In fact, liquid transportation is an active area for researchers. Brochard reported the motions of droplets on solid surfaces induced by chemical or thermal gradients, in which the Marangoni effect plays an important role (Brochard 1989). Whitesides & Chaudhury first reported the uphill movement of a water droplet on a surface of gradient wettability (Whitesides & Chaudhury 1992). The self-motion of the droplet is driven by the imbalanced forces due to the gradient surface tension acting on the solid-liquid contact lines of the droplet. Lorenceau & Quéré reported the self-propelling behavior of wetting silicone oil droplets on a conical fiber, in which the driving force is shown to be a gradient of the Laplace pressure of the asymmetric droplets (Lorenceau & Quéré 2004).

Usually, in order to transport liquids across solid surfaces, the construction of a gradient in the interfacial tension is critical at the front and rear ends of the droplet acting at the liquid-solid-vapor interface. It has been demonstrated that surface tension heterogeneity-induced driving force can be used to guide water motion on flat surfaces. Wang et al. reported a novel method to prepare a one-way oil-transport fabric and its application in detecting liquid surface tension (Wang et al. 2015). This functional fabric was prepared by a two-step coating process to apply flowerlike ZnO nanorods, fluorinated decyl polyhedral oligomeric silsesquioxanes, and hydrolyzed FAS on a fabric substrate. Upon one-sided UV irradiation, the coated fabric shows a one-way transport feature that allows oil fluid transport automatically from the unirradiated side to the UV-irradiated surface, but it stops fluid transport in the opposite direction. The fabric still maintains high superhydrophobicity after UV treatment. The one-way fluid transport takes place only for oil fluids with a specific surface tension value, and the fluid selectivity is dependent on the UV treatment time. Changing the UV irradiation time from 6 to 30 hours broadened the one-way transport for fluids with surface tensions from around 22.3 mN/m to a range of 22.3–56.7 mN/m. For microfluidic systems, it is important to precisely control the liquid flow within microchannels.

Research involving superhydrophilic and superhydrophobic surfaces actively started only about a decade ago. Since then, many different techniques and materials to produce both types of surfaces have been developed, and most of the research results have already been commercialized or show a great potential application for solving practical problems. Ingenious integration of different special wettabilities provides effective ways of uniting the advantages of the wettabilities, pioneering a new way to develop advanced interface materials. Over the last few years, a number of methods allowing for the fabrication of superhydrophilic-superhydrophobic patterned substrates have been introduced. The key to integrating different super wettabilities is to take advantage of the surface energy difference between two types of surfaces. As shown in this progress report, the trend is now shifting toward exploring better preparation methods and the development of new applications that use the unique properties of such hybrid patterns.

Although the current micropatterned surfaces have shown enhanced efficiency in vapor/fog collection, part of the theory and mechanism still remains unclear and should be further investigated. Detailed fluid behaviors, particularly tiny-droplet generation and collection on wettability boundaries should be observed microscopically to determine the functions of wettability difference. Deeper understanding of fluid behaviors at the wettability boundary of hybrid surface should instruct researchers to develop advanced superwettability integration. In addition, the application of research results requires us to consider many practical factors that are often overlooked in the research process, such as technical applicability and scalability, ease of setup, cost effectiveness, stability, and durability. With the continuous progress of science and engineering technology, we believe that superhydrophilic-superhydrophobic micropatterns have a promising future in improving the livability and sustainability of our plant.

This project is supported by the Natural Science Foundation of China (50772131) and National High-tech R&D Program of China (863 Program) (2001AA322100), a grant from the Fundamental Research Funds for the Central Universities (2010YJ05) and the Key projects of the Ministry of Education (106086).

Al-khayat
O.
Hong
J. K.
Beck
D. M.
Minett
A. I.
Neto
C.
2017
Patterned polymer coatings increase the efficiency of dew harvesting
.
ACS Applied Materials & Interfaces
9
(
15
),
13676
13684
.
Bai
H.
Tian
X. L.
Zheng
Y. M.
Ju
J.
Zhao
Y.
Jiang
L.
2010
Direction controlled driving of tiny water drops on bioinspired artificial spider silks
.
Advanced Materials
22
(
48
),
5521
5525
.
Bai
H.
Wang
L.
Ju
J.
Ju
R. Z.
Zheng
Y. M.
Jiang
L.
2014
Efficient water collection on integrative bioinspired surfaces with star-shaped wettability patterns
.
Advanced Materials
26
(
29
),
5025
5030
.
Beysens
D.
2006
Dew nucleation and growth
.
Comptes Rendus Physique
7
,
1082
1100
.
Briscoe
B. J.
Galvin
K. P.
1906
Growth with coalescence during condensation
.
Physical Review A
43
(
4
),
1906
1917
.
Brusatin
G.
Guglielmi
M.
Innocenzi
P.
Martucci
A.
Scarinci
G.
2000
Materials for photonic applications from sol-gel
.
Journal of Electroceramics
4
(
1
),
151
165
.
Cao
M. Y.
Jiang
L.
2016
Super-wettabilities integration: concepts, design and applications
.
Surface Innovations
4
(
4
),
180
194
.
Cao
M. Y.
Xiao
J. S.
Yu
C. M.
Li
K.
Jiang
L.
2015
Hydrophobic/hydrophilic cooperative Janus system for enhancement of fog collection
.
Small
11
(
34
),
4379
4384
.
Chang
F. M.
Hong
S. J.
2010
Wetting invasion and retreat across a corner boundary
.
Journal of Physical Chemistry C
114
(
3
),
1615
1621
.
Chang
F. M.
Sheng
Y. J.
Tsao
H. K.
2009
High contact angle hysteresis of superhydrophobic surfaces: hydrophobic defects
.
Applied Physics Letters
95
(
6
),
204107
.
Chaturvedi
N. D.
Manan
Z. A.
Alwi
S. R. W.
Bandyopadhyay
S.
2016
Effect of multiple water resources in a flexible-schedule batch water network
.
Journal of Cleaner Production
125
,
245
252
.
Chen
D. L.
Li
J.
Zhao
J. Y.
Guo
J.
Zhang
S. B.
Ambreen
T. A.
Sherazi
T. A.
Li
S. H.
2018
Bioinspired superhydrophilic-hydrophobic integrated surface with conical pattern-shape for self-driven fog collection
.
Journal of Colloid and Interface Science
530
,
274
281
.
Cho
H. D.
Park
B.
Kim
K.
Lee
S. M.
Hwang
W. B.
2017
A large-scale water-harvesting device with β-Al(OH)3 microcone arrays by simple hydrothermal synthesis
.
Journal of Materials Chemistry
5
(
48
),
25328
25337
.
Choi
S. J.
Suh
K. Y.
Lee
H. H.
2008
Direct UV-replica molding of biomimetic hierarchical structure for selective wetting
.
Journal of the American Chemical Society
130
(
20
),
6312
6313
.
Colusso
E.
Martucci
A.
Neto
C.
2019
Fabrication of biomimetic micropatterned surfaces by sol-gel dewetting
.
Advanced Materials Interfaces
6
(
4
),
1801629
.
Comanns
P.
2018
Passive water collection with the integument: mechanisms and their biomimetic potential
.
The Journal of Experimental Biology
221
(
10
),
1
13
.
jeb 153130
.
Dolatshahipirouz
A.
Nikkhah
M.
Gaharwar
A. K.
Hashmi
B.
Guermain
E.
2014
A combinatorial cell-laden gel microarray for inducing osteogenic differentiation of human mesenchymal stem cells
.
Scientific Reports
4
(
1
),
3896
3896
.
Figueira
R. B.
Silva
C. J. R.
Pereira
E. V.
2015
Organic-inorganic hybrid sol-gel coatings for metal corrosion protection: a review of recent progress
.
Journal of Coatings Technology and Research
12
,
1
35
.
Fritzmann
C.
Löwenberg
J.
Wintgens
T.
2007
State-of-the-art reverse osmosis desalination
.
Desalination
216
,
1
76
.
Gao
Y.
Wang
J.
Mou
X. F.
Cai
Z. S.
2018a
Textile-inspired methodology toward asymmetric fabric based on weft-backed weave for oil/water separation
.
Journal of Materials Science
53
(
6
),
4683
4692
.
Gao
Y.
Wang
J.
Xia
W.
Mou
X. F.
Cai
Z. S.
2018b
Reusable Hydrophilic–superhydrophobic patterned weft backed woven fabric for high-efficiency water-harvesting application
.
ACS Sustainable Chemistry & Engineering
6
(
6
),
7216
7220
.
Garrod
R. P.
Harris
L. G.
Schofield
W. C. E.
Gettrick
J. M.
Ward
L. J.
Teare
D. O. H.
Badyal
J. P. S.
2007
Mimicking a Stenocara beetle's back for microcondensation using plasmachemical patterned superhydrophobic-superhydrophilic surfaces
.
Langmuir
23
(
2
),
689
693
.
Ge
M. Z.
Li
Q.
Cao
C. Y.
Huang
J. Y.
Li
S. H.
Zhang
S. N.
Chen
Z.
Zhang
K. Q.
Al-Deyab
S.
Lai
Y. K.
2017
One-dimensional TiO2 nanotube photocatalysts for solar water splitting
.
Advanced Science
4
(
1
),
1600152
1600152
.
Gilbertson
L. M.
Zimmerman
J. B.
Plata
D. L.
Hutchison
J. E.
Anastas
P. T.
2015
Designing nanomaterials to maximize performance and minimize undesirable implications guided by the principles of green chemistry
.
Chemical Society Reviews
44
,
5758
5777
.
Gohari
A.
Eslamian
S.
Mirchi
A.
Jahangir
A. K.
Bavani
A. M.
Madani
K.
2012
Climate change impacts on crop production in Iran's zayandeh-rud river basin
.
Science of The Total Environment
442C
,
405
419
.
Gorjian
S.
Ghobadian
B.
2015
Solar desalination: a sustainable solution to water crisis in Iran
.
Renewable & Sustainable Energy Reviews
48
(
48
),
571
584
.
Gorjian
S.
Ghobadian
B.
Hashjin
T. T.
Banakar
A.
2014
Experimental investigation and performance analysis of a point-focus parabolic solar still
.
Desalination
352
,
1
17
.
Gou
N.
Yuan
S. H.
Lan
J. Q.
Gao
C.
Alshawabkeh
A. N.
Gu
A. Z.
2014
A quantitative toxicogenomics assay reveals the evolution and nature of toxicity during the transformation of environmental pollutants
.
Environmental Science & Technology
48
(
15
),
8855
8863
.
Guadarrama-Cetina
J.
Mongruel
A.
Medici
M.-G.
Baquero
E.
Parker
A. R.
Milimouk-Melnytchuk
I.
González-Viñas
W.
Beysens
D.
2014
Dew condensation on desert beetle skin
.
European Physical Journal E
37
(
11
),
1
6
.
Hamdan
M. A.
Refaat
A. A.
Anwar
E. A.
Shallaly
N. A.
2015
Source of the aeolian dune sand of Toshka area, southeastern Western Desert
.
Aeolian Research
17
,
275
289
.
Hamilton
W. J.
Seely
M. K.
1976
Fog basking by the Namib Desert beetle, Onymacris unguicularis
.
Nature
262
(
5566
),
284
285
.
Hanikel
N.
Prévot
M. S.
Yaghi
O. M.
2020
MOF water harvesters
.
Nature Nanotechnology
15
,
348
355
.
Hanna-Attisha
M.
LaChance
J.
Casey Sadler
R.
Champney Schnepp
A.
2016
Elevated blood lead levels in children associated with the flint drinking water crisis: a spatial analysis of risk and public health response
.
American Journal of Public Health
106
(
2
),
283
290
.
Hybel
A. M.
Godskesen
B.
Rygaard
M.
2015
Quantifying freshwater impacts of groundwater abstraction in water supplies
.
Journal of Environmental Management
160
,
90
97
.
Jankowitz
W. J.
Rooyen
M. W. V.
Shaw
D.
Kaumba
J. S.
2008
Mysterious circles in the Namib Desert
.
South African Journal of Botany
74
(
2
),
332
334
.
Jiang
J. K.
Bao
B.
Li
M. Z.
Sun
J. Z.
Zhang
C.
Li
Y.
2016
Fabrication of transparent multilayer circuits by inkjet printing
.
Advanced Materials
28
(
7
),
1420
1426
.
Jörn
B.
Jörn
K.
Hohm
S.
Rosenfeld
A.
2012
Femtosecond laser-induced periodic surface structures
.
Journal of Laser Applications
24
(
4
),
T. A.
042006
.
Ju
J.
Zheng
Y. M.
2014
Bioinspired one-dimensional materials for directional liquid transport
.
Accounts of Chemical Research
47
(
8
),
2342
2352
.
Ju
J.
Bai
H.
Zheng
Y. M.
Zhao
T. Y.
Fang
R. C.
Jiang
L.
2012
A multi-structural and multi-functional integrated fog collection system in cactus
.
Nature Communications
3
(
1
),
1247
1247
.
Jurado
A.
Vàzquez-suñé
E.
Carrera
J.
Alda
M. L.
Pujades
E.
Barceló
D.
2012
Emerging organic contaminants in groundwater in Spain: a review of sources, recent occurrence and fate in a European context
.
Science of The Total Environment
440
,
82
94
.
Kang
S. M.
You
I.
Cho
W. K.
Shon
H. K.
Lee
T. G.
Chol
I. S.
Karp
J. M.
Lee
H.
2010
One-step modification of superhydrophobic surfaces by a mussel-inspired polymer coating
.
Angewandte Chemie
49
(
49
),
9401
9404
.
Khawaji
A. D.
Kutubkhanah
I. K.
Wie
J. M.
2007
Advances in seawater desalination technologies
.
Desalination
221
(
221
),
47
69
.
Lancaster
J.
Lancaster
N.
Seely
M. K.
1984
Climate of the central Namib desert
.
Madoqua
14
,
5
61
.
Lee
G. R.
Crayston
J. A.
1993
Sol-gel processing of transition-metal alkoxides for electronics
.
Advanced Materials
5
(
6
),
434
442
.
Lee
H.
Dellatore
S. M.
Miller
W. M.
Messersmith
P. B.
2007
Mussel-inspired surface chemistry for multifunctional coatings
.
Science
318
(
5849
),
426
430
.
Lee
S. H.
Lee
J. H.
Park
C. W.
Lee
C. Y.
Kim
K. S.
Tahk
D. H.
2014
Continuous fabrication of bio-inspired water collecting surface via roll-type photolithography
.
International Journal of Precision Engineering and Manufacturing-Green Technology
1
(
2
),
119
124
.
Li
J. S. S.
Ueda
E.
Nallapaneni
A.
Li
L. X. X.
Levkin
P. A.
2012
Printable superhydrophilic–superhydrophobic micropatterns based on supported lipid layers
.
Langmuir
28
(
22
),
8286
8291
.
Li
X. H.
Wang
J.
Liu
X.
Liu
L.
Cha
D.
Han
Y.
2019
Direct imaging of tunable crystal surface structures of mof mil-101 using high-resolution electron microscopy
.
Journal of the American Chemical Society
141
(
30
),
12021
12028
.
Logan
M. W.
Spencer
L.
Xia
Z. Y.
2020
Reversible atmospheric water harvesting using metal-organic frameworks
.
Scientific Reports
10
(
1
),
1
11
.
Lorenceau
E.
Quéré
D.
2004
Drops on a conical wire
.
Journal of Fluid Mechanics
510
,
29
45
.
Lutchmiah
K.
Verliefde
A. R. D.
Roest
K.
Rietveld
L. C.
Cornelissen
E. R.
2014
Forward osmosis niches in seawater desalination and wastewater reuse
.
Water Research
58
,
179
197
.
Lv
C. J.
Chen
C.
Chuang
Y. C.
Tseng
F. G.
Yin
Y. J.
Grey
F.
2014
Substrate curvature gradient drives rapid droplet motion
.
Physical Review Letters
113
(
2
),
026101
.
Manuel
G.
Thickett
S. C.
Telford
A. M.
Easton
C. D.
Meagher
L.
Neto
C.
2014
Protein micropatterns by PEG grafting on dewetted PLGA films
.
Langmuir
30
(
39
),
11714
11722
.
Medici
M. G.
Mongruel
A.
Royon
L.
Beysens
D.
2014
Edge effects on water droplet condensation
.
Physical Review E
90
(
6
),
062403
.
Mekonnen
M. M.
Hoekstra
A. Y.
2016
Four billion people facing severe water scarcity
.
Science Advances
2
(
2
),
e1500323
.
Moazzam
P.
Tavassoli
H.
Razmjou
A.
Warkiani
M. E.
Asania
M.
2018
Mist harvesting using bioinspired polydopamine coating and microfabrication technology
.
Desalination
429
,
111
118
.
Murray
I. W.
Fuller
A.
Lease
H. M.
Mitchell
D.
Hetem
P. S.
2016
Low field metabolic rates for geckos of the genus Rhoptropus may not be surprising
.
Journal of Arid Environments
124
,
225
232
.
Nishimoto
S.
Becchaku
M.
Kameshima
Y.
Hayakawa
S.
Osaka
A.
Miyake
M.
2014
TiO2-based superhydrophobic-superhydrophilic pattern with an extremely high wettability contrast
.
Thin Solid Films
558
,
221
226
.
Oliveira
M. B.
Neto
A. I.
Correia
C. R.
Rial-Hermida
M. I.
Alvarez-Lorenzo
C.
Mano
J. F.
2014a
Superhydrophobic chips for cell spheroids high-throughput generation and drug screening
.
ACS Applied Materials & Interfaces
6
(
12
),
9488
9485
.
Oliveira
S. M.
Alves
N. M.
Mano
J. F.
2014b
Cell interactions with superhydrophilic and superhydrophobic surfaces
.
Journal of Adhesion Science and Technology
28
,
843
863
.
Oliver
J. F.
Huh
C.
Mason
S. G.
1977
Resistance to spreading of liquids by sharp edges
.
Journal of Colloid and Interface Science
59
(
3
),
568
581
.
Pan
T. T.
Yang
K. J.
Han
Y.
2020
Recent progress of atmospheric water harvesting using metal-organic frameworks
.
Chemical Research in Chinese Universities
36
(
1
),
33
40
.
Park
K. C.
Kim
P.
Grinthal
A.
He
N.
Fox
D.
Weaver
J. C.
Aizenberg
J.
2016
Condensation on slippery asymmetric bumps
.
Nature
531
(
7592
),
78
82
.
Parker
A. R.
Lawrence
C. R.
2001
Water capture by a desert beetle
.
Nature
414
(
6859
),
33
34
.
Peisajovich
S. G.
2012
Evolutionary synthetic biology
.
ACS Synthetic Biology
1
(
6
),
199
210
.
Pietruszka
R. D.
Seely
M. K.
1985
Predictability of two moisture sources in the Namib Desert
.
South African Journal of Science
81
,
682
685
.
Piret
G.
Desmet
R.
Diesis
E.
Drobecq
H.
Segers
J.
Rouanet
C.
2010
Chips from chips: application to the study of antibody responses to methylated proteins
.
Journal of Proteome Research
9
(
12
),
6467
6478
.
Popova
A. A.
Demir
K.
Hartanto
T. G.
Schmitt
E.
Levkin
P. A.
2016
Droplet-microarray on superhydrophobic–superhydrophilic patterns for high-throughput live cell screenings
.
RSC Advances
6
(
44
),
38263
38276
.
Popova
A.
Depew
C.
Permana
K. M.
2017
Evaluation of the droplet-microarray platform for high-throughput screening of suspension cells
.
Journal of Laboratory Automation
22
(
2
),
163
175
.
Ridoutt
B. G.
Pfister
S.
2010
Reducing humanity's water footprint
.
Environmental Science & Technology
44
(
16
),
6019
6021
.
Rizzello
L.
Shankar
S. S.
Fragouli
D.
Athanassiou
A.
Cingolani
R.
Pompa
P. P.
2009
Microscale patterning of hydrophobic/hydrophilic surfaces by spatially controlled galvanic displacement reactions
.
Langmuir
25
(
11
),
6019
6023
.
Rohr
T.
Hilder
E. F.
Donovan
J. J.
Svec
F.
Frechet
J. M.
2003
Photografting and the control of surface chemistry in three-dimensional porous polymer monoliths
.
Macromolecules
36
(
5
),
1677
1684
.
Schemenauer
R. S.
Cereceda
P.
1991
Fog-water collection in arid coastal locations
.
AMBIO: A Journal of the Human Environment
20
(
7
),
303
308
.
Schemenauer
R. S.
Cereceda
P.
1994
Fog collection's role in water planning for developing countries
.
Natural Resources Forum
18
(
2
),
91
100
.
Schoenecker
P. M.
Carson
C. G.
Flemming
C. J. J.
2012
Effect of water adsorption on retention of structure and surface area of metal-organic frameworks
.
Industrial & Engineering Chemistry Research
51
(
18
),
6513
6519
.
Song
W. L.
Veiga
D. D.
Custódio
C. A.
Mano
J. F.
2009
Bioinspired degradable substrates with extreme wettability properties
.
Advanced Materials
21
(
18
),
1830
1834
.
Suh
B. L.
Chong
B. S.
Kim
J.
2019
Photochemically induced water harvesting in metal–organic framework
.
ACS Sustainable Chemistry & Engineering
7
(
19
),
15854
15859
.
Sun
J. Z.
Bao
B.
He
M.
Zhou
H. H.
Song
Y. L.
2015
Recent advances in controlling the depositing morphologies of inkjet droplets
.
ACS Applied Materials & Interfaces
7
(
51
),
28086
28099
.
Telford
A. M.
Meagher
L.
Glattauer
V.
Gengenbach
T. R.
Easton
C. D.
Neto
C.
2012
Micropatterning of polymer brushes: grafting from dewetting polymer films for biological applications
.
Biomacromolecules
13
(
9
),
2989
2996
.
Telford
A. M.
Thickett
S. C.
Neto
C.
2017
Functional patterned coatings by thin polymer film dewetting
.
Journal of Colloid and Interface Science
507
,
453
469
.
Tserepi
A.
Gogolides
E.
Bourkoula
A.
Kanioura
A.
Kokkoris
G.
Petrou
P. S.
2016
Plasma nanotextured polymeric surfaces for controlling cell attachment and proliferation: a short review
.
Plasma Chemistry and Plasma Processing
36
(
1
),
107
120
.
Varanasi
K. K.
Hsu
M.
Bhate
N.
Wang
W. S.
Deng
T.
2009
Spatial control in the heterogeneous nucleation of water
.
Applied Physics Letters
95
(
9
),
094101
.
Vekilov
P. G.
2010
Nucleation
.
Crystal Growth & Design
10
(
12
),
5007
5019
.
Vengosh
A.
Jackson
R. B.
Nathaniel
W.
Darrah
T. H.
Kondash
A.
2014
A critical review of the risks to water resources from unconventional shale gas development and hydraulic fracturing in the United States
.
Environmental Science & Technology
48
(
15
),
8334
8348
.
Wang
H. X.
Zhou
H.
Yang
W. D.
Zhao
Y.
Fang
J.
Lin
T.
2015
Selective, spontaneous one-way oil-transport fabrics and their novel use for gauging liquid surface tension
.
ACS Applied Materials & Interfaces
7
(
41
),
22874
22880
.
Wang
M.
Liu
Q.
Zhang
H. R.
Wang
C.
Wang
L.
Xiang
B. X.
2017a
Laser direct writing of tree-shaped hierarchical cones on a superhydrophobic film for high-efficiency water collection
.
ACS Applied Materials & Interfaces
9
(
34
),
29248
29254
.
Wang
S. F.
Liu
M. S.
Feng
Y.
Huynh
S. H.
Ng
T. W.
Gu
F.
2017b
Bioinspired hierarchical copper oxide surfaces for rapid dropwise condensation
.
Journal of Materials Chemistry
5
(
40
),
21422
21428
.
Whitesides
G. M.
Chaudhury
M. K.
1992
How to make water run uphill
.
Science
256
(
5063
),
1539
1541
.
Woodyer
R.
Chen
W.
Zhao
H. M.
2004
Outrunning nature: directed evolution of superior biocatalysts
.
Journal of Chemical Education
81
(
1
),
126
133
.
Young
T.
1805
An essay on the cohesion of fluids
.
Philosophical Transactions of the Royal Society
95
,
65
87
.
Yu
Z. W.
Yun
F. F.
Wang
Y. Q.
Li
Y.
Dou
S. X.
Liu
K. S.
2017
Desert beetle-inspired superwettable patterned surfaces for water harvesting
.
Small
13
(
36
),
1701403
.
Zahner
D.
Abagat
J.
Svec
F.
Frechet
J. M.
Levkin
P. A.
2011
A facile approach to superhydrophilic-superhydrophobic patterns in porous polymer films
.
Advanced Materials
23
(
27
),
3030
3034
.
Zhai
L.
Berg
M. C.
Cebeci
F. C.
Kim
Y.
Milwid
J. M.
2006
Patterned superhydrophobic surfaces: toward a synthetic mimic of the Namib Desert beetle
.
Nano Letters
6
(
6
),
1213
1217
.
Zhang
L. B.
Wu
J. B.
Hedhili
M. N.
Yang
X. L.
Wang
P.
2015
Inkjet printing for direct micropatterning of a superhydrophobic surface: toward biomimetic fog harvesting surfaces
.
Journal of Materials Chemistry
3
(
6
),
2844
2852
.
Zhao
S. F.
Zou
L. D.
Tang
C. Y.
Mulcahy
D.
2012
Recent developments in forward osmosis: opportunities and challenges
.
Journal of Membrane Science
396
,
1
21
.
Zhu
X. T.
Zhang
Z. Z.
Men
X. H.
Yang
J.
Xu
X. H.
2010
Rapid formation of superhydrophobic surfaces with fast response wettability transition
.
ACS Applied Materials & Interfaces
2
(
12
),
3636
3641
.
Zhu
H.
Guo
Z.
Liu
W.
2016
Biomimetic water-collecting materials inspired by nature
.
Chemical Communications
52
(
20
),
3863
3879
.